1. Field of the Invention
The invention relates to semiconductor manufacturing, especially to mask layout design.
2. Brief Discussion of the Related Art
A typical semiconductor design process includes numerous steps. A circuit designer first prepares a schematic diagram that includes logical connections between logic elements that together form an integrated circuit. The schematic diagram is then verified to determine if the logic elements and associated logical connections perform a desired function. After the circuit is verified, the schematic diagram is converted into a mask layout database that includes a series of polygons. The polygons may represent the logic elements and the logical connections contained in the schematic diagram. The mask layout database is used to generate masks, also known as reticles, that may be used for patterning different layers of the integrated circuit onto a semiconductor wafer.
Typically, the mask layout database is created manually by a layout designer or automatically by a synthesis tool. In a 0.13 micron (μm) or below manufacturing process, the layout designer or synthesis tool may use thousands of design rules to create the mask layout database. Conventionally, the design rule specifies a width/space limit for patterns in a mask layout. For example, the design rule specifies that patterns in a layout cannot be narrower than 0.13 μm. A typical mask layout, however, comprises patterns of different shapes, widths, and arrangements, such as a dense, isolated, L-shape, T-shape, line-end, or other pattern. When patterns of different characteristics are integrated into a mask layout, a reduced process window for a manufacturing process may be result. For example, a pattern comprising 0.13 μm dense line/space may have a 0.6 μm depth of focus, while a pattern comprising 0.13 μm isolated line may have a 0.3 μm depth of focus. Therefore, when patterns comprising 0.13 μm dense line/space and 0.13 μm isolated line (or isolated space) are integrated into a mask layout, a corresponding lithography process window may be narrowed down to 0.3 μm, which is too strict for mass production.